Anopheles stephensi larval habitat superproductivity and its relevance for larval source management in Ethiopia

The invasion of Africa by Anopheles stephensi poses a significant threat to malaria elimination. Unlike African Anopheline species, An. stephensi may be less impacted by rainfall seasonality due to its predominance within artificial containers. To test this assumption, the seasonal transition of an established population from eastern Ethiopia between rainy and dry periods was quantified. Secondary objectives included the theoretical quantification of the impact of habitat seasonality on the efficacy of larval control, and the study of habitat overlap between An. stephensi and Aedes aegypti.

The study was conducted in the city of Kebri Dehar, Somali Region of Ethiopia. Standard dipping was conducted in 100 houses during the peak rainy season (November 2020). All habitats with water at that survey were followed monthly until the peak of the dry season (February 2021). Positivity and productivity of An. stephensi and Ae. aegypti were quantified. Negative binomial and Pareto fraction analyses characterized heterogeneity in productivity by habitat type. A two-patch metapopulation model quantified the impact of rainfall seasonality and heterogeneity in larval habitat productivity on the entomological impact of a larval source management campaign.

As the dry season progressed, An. stephensi productivity significantly concentrated in large water reservoirs (for drinking and construction). Larval productivity was significantly overdispersed (parameter k of negative binomial distribution ranging between 0.28 and 0.43). Pareto distribution analyses estimated that 77% of all larvae originated from 23% of the sites. Such superproductive sites were primarily water cisterns used for residential or construction purposes. The most productive An. stephensi habitats were not so for Ae. aegypti, which infested primarily tires and small containers. The metapopulation model predicted that larval control targeted on superproductive water reservoirs, when implemented at coverages higher than 60%, may lead to An. stephensi elimination.

This study highlights the role of environmental variability in regulating An. stephensi populations and opens the window for the deployment of control strategies that exploit major mosquito population bottlenecks. The partial overlap between An. stephensi and Ae. aegypti, particularly during the dry season, point to challenges to control both species if only large containers are treated.